The present invention relates to high-strength, melt-processed films having a fibrous surface, and methods of making the same. The films of the invention can be prepared by imparting fluid energy, typically in the form of ultrasound or high-pressure water jets, to an oriented, melt processed film comprising a semicrystalline polymer and void initiating component. Fibrous films of the invention find use as tape backings, paper-like substrates for printing or graphics, filters, fibrous mats and thermal and acoustical insulation.
Polymeric fibers have been known essentially since the beginnings of commercial polymer development. The production of polymer fibers from polymer films is also well known. In particular, the ease with which films produce fibers (i.e., fibrillate) can be correlated to the degree of molecular orientation of the polymer fibrils that make up the film.
Porous and microporous films have been prepared by extruding thermoplastic polymers, followed by orientation of the film. U.S. Pat. No. 5,811,493 (Kent) describes extruding a composition comprising a polyester continuous phase, a thermoplastic polyolefin discrete phase and a polyester-polyether diblock compatibilizer, followed by orientation. U.S. Pat. No. 4,921,652 (Tsuji et al.) describes melt-molding a blend of an inorganic fine powder with a polyolefin, followed by stretching. U.S. Pat. No. 4,377,616 (Ashcroft et al.) describes a biaxially oriented polymer film comprising a voided, thermoplastic matrix material where the voids are created within the matrix by inclusion of spherical void-initiating solid particles which are incompatible with the matrix material.
Orientation of semicrystalline polymeric films and fibers has been accomplished in numerous ways, including melt spinning, melt transformation (co)extrusion, solid state coextrusion, gel drawing, solid state rolling, die drawing, solid state drawing, and roll-trusion, among others. Each of these methods has been successful in preparing oriented, high modulus polymer fibers and films. Most solid-state processing methods have been limited to slow production rates, on the order of a few cm/min. Methods involving gel drawing can be fast, but require additional solvent-handling steps. A combination of rolling and drawing solid polymer sheets, particularly polyolefin sheets, has been described in which a polymer billet is deformed biaxially in a two-roll calender then additionally drawn in length (i.e., the machine direction). Methods that relate to other web handling equipment have been used to achieve molecular orientation, including an initial nip or calender step followed by stretching in both the machine direction or transversely to the film length.
The present invention is directed to novel oriented films having at least one fibrous surface, said films comprising a melt-processed immiscible mixture of a semicrystalline polymer component and a void-initiating component. The fibrous surface of the film advantageously provides a greater surface area making films especially useful in applications such as ink-receptive substrates, wipes, paper-like films and as backings for tapes. The surface area is generally greater than about 0.25 m2/gram, typically about 0.5 to 30 m2/g.
The present invention is further directed toward the preparation of oriented films having a fibrous surface by the steps of providing an oriented polymer film comprising an immiscible mixture of a crystalline polymer component and a void-initiating component, stretching the film along at least one major axis (uniaxial orientation) to impart a voided morphology thereto, optionally stretching the film along a second major axis (biaxial orientation), and then microfibrillating the voided film by imparting sufficient fluid energy thereto.
The films may be uniaxially oriented to produce a fibrous surface having polymeric microfibers of average effective diameter of less than 20 microns, generally from 0.01 to 10 microns, and substantially rectangular in cross section, having a transverse aspect ratio (width to thickness) of from 1.5:1 to 20:1. Further, due to their highly oriented morphology, the microfibers of the present invention, produced from uniaxially oriented films, have very high modulus making them especially useful as reinforcing fibers in thermoset resin and concrete. Alternatively, the films may be biaxially oriented to produce a fibrous surface of fibrous flakes that are thin in cross-section, in comparison to the width and lengths, and irregular in shape. The fibrous flakes impart a large surface area to the film are particularly useful as adherent surfaces for bonding or fastening.
Advantageously the process of the invention is capable of high rates of production, is suitable as an industrial process and uses readily available polymers. Further, the use of immiscible mixtures allows for microfibrillation of the film surfaces with less imparted energy as compared to the microfibrillation of a single component polymer film. The fibers and fibrous articles of this invention, having extremely small fiber diameter and both high strength and modulus, are useful as tape backings, strapping materials, films with unique optical properties and high surface area, low density reinforcements for thermosets, impact modifiers or crack propagation prevention in matrices such as concrete, as paper-like substrates for printing or graphics, and as fibrillar forms (dental floss or nonwovens, for example).